Method and composition for treating aluminum and aluminum alloys



Patented Apr. 22, 1952 UNITED STATES PATENT OFFICE METHOD AND COMPOSITION FOR TREAT- ING ALUMINUM. AND ALUMINUM ALLOYS No Drawing. Application October 26, 1950, Serial No. 192,373

15 Claims.

This invention relates to the brightening of aluminum and aluminum alloys. More particularly, the invention relates to a composition and method for chemical brightening of aluminum and its alloys.

This application is a continuation-in-part of my copending application Serial No. 101,694, filed June 27, 1949.

Although aluminum is ordinarily considered a bright metal, it often presents a dull or mattelike finish due to the oxide films formed on its surfaces during processing. Many aluminum and aluminum alloy products are fabricated from mill finished sheet having such a characteristic dull appearance, which leads to a demand by fabricators for means to impart a bright, lustrous finish to their products. One widely used method of polishing aluminum and its alloys is by mechanical buffing with a suitable abrasive. However, this mechanical polishing is expensive and is not easily adaptable to articles having intricate shapes and inaccessible surfaces. Other methods known as electropolishing and electrobi'lghtening involve subjecting the aluminum article to an electrolytic treatment and produce desirable lustrous finishes. However, these methods are both slow and expensive.

It is, therefore, a primary object and purpose of the invention to provide a composition and method for the chemical brightening or polishing of aluminum. A further object is to provide a chemical solution or bright dip composition for aluminum and aluminum alloys which is characterized by its economy in use, short immersion time required to obtain the desired effect, and ease of application. Another object is to produce on aluminum and aluminum alloy articles polished or brightened surfaces by means of a chemical bright dip composition and process which effectively removes the dulling oxide films from the metallic surfaces and imparts thereto. a high luster.

Chemical brightening of aluminum has previously been accomplished using a combination of nitric and hydrofluoric acids. The hydrofluoric acid readily dissolves any oxide film and However, the etching or pitting action of the HF proved, extremely difiicult to control regardless. of the concentration or temperature employed and uniform results could not be consistently obtained at a favorable rate of reaction.

It has been discoveredaccording to thepresent invention that uniform and superior chemical brightening of aluminum and aluminum alloys is obtained with a bright dip composition comprising an aqueous acid solution containing in addition to hydrogen, nitrate and fluoride ions, phosphate and ammonium ions.

The presence of the phosphate and ammonium ions appears to exert a modifying influence on the action of the acid bath containing fluoride ions. Previously, the hydrofluoric acid, although effective in removing the dulling oxide film, caused pitting or uneven etching of the metallic surfaces which prevented the production of the desired specular-like bright finish. Reduction in concentration of the hydrofluoric acid merely resulted in a decrease in the pitting effect, but at the same time it appreciably lowered the rate of reaction of the solution, thus increasing immersion time. It was found that by the introduction of phosphate ions and ammonium ions the concentration of hydrofluoric acid could be maintained sufficiently high to give optimum rate of action while overetching or pitting was substantially eliminated. Furthermore, the presence of phosphate ions prevented formation of smut on the metal surface bein treated, and a brighter smoother surface was produced.

It was then discovered that the specular reflectivity of the brightened metal surfaces was increased by the presence of the cations of a metal more electropositive than aluminum in the acid aqueous solution. It had been noted that aluminum and aluminum alloys substantially free of copper, for example, when that element was present as an incidental impurity, although satisfactorily brightened, had a thin white film on the brightened surface which impaired somewhat the specular reflectivity. Incorporation of the cations of a metal more electropositive than aluminum in the bath eliminated the film and produced optimum specular brightness. This beneficial effect was noted in all cases, but in diminishing degree as the copper content of the alloy increased. It is believed in such cases that a portion of the copper content of the alloy surface dissolved out and effectively eliminated the film formation and imparted optimum specular brightness in the first instance. Accordingly, the presence of the cations of a metal more electropositive than aluminum is recommended for optimum results, particular- 1y when the metal treated is low in copper, for example, about .05% or below. However, it is to be understood that the bath functions to produce brightened surfaces without this constituent.

The term electropositive as used in the specification and claims is based upon the definition and values of single electrode potentials on the hydrogen scale given in the Chemical Engineers Handbook, John H. Perry, editorin-chief, 2nd ed., eighth impression (1941), McGraw-Hill Book Company), pages 2746 to 2748, inclusive.

In general, it was noted that those metals having an electrode potential sufficiently more positive than aluminum that the galvanic couple resulting had a relatively high EMF (compared, for example to the AlZn couple) produced the most advantageous results. Accordingly metals such as copper, silver, gold and platinum are preferred, copper being the preferred species on the basis of its optimum effect, cost and availability. Other metals are productive of highly satisfactory results among which may be mentioned iron, nickel and cobalt.

The metal cation may advantageously be introduced in the form of a soluble metal salt, that is, soluble in the acid aqueous bath. The anion of the metal may be any anion compatible with the other constituents of the solution. It is preferred, where convenient to utilize the nitrates in order to avoid undue complexity of the solution. Copper nitrate is accordingly the preferred species.

The amount of the electropositive metal may be very small while effectively increasing specular brightness and eliminating the film when the latter is present. The concentration of metal cation may be expressed as that produced by or stoichiometrically equivalent to an amount of a salt of the selected metal. The salt may be present in amount as low as 0.001% by weight, although much larger amounts may be used where bril-' liance of the metal is the primary consideration and the formation of a metallic smut on the brightened surface and its removal is not objectionable. Accordingly amounts of metal salt up to about 1% are not excessive where the thin loosely adherent metal smut deposited on the aluminum surface is subsequently removed by mechanical wiping or by dipping in dilute or even concentrated nitric acid. Where the work is to be anodized after brightening, the smut dissolves off the surface during the electrolytic treatment. Where no smut is desirable, the metal cation concentration is confined to the lower range of amounts, that is, below the amount at which smut first forms. This amount varies with the composition of the bath and the metal treated, and, as an example, did not exceed about .0015% of copper nitrate'calculated as CU(NO3)2'3H2O when treating an aluminum alloy containing only about 05% copper.

The brightness of the surface of the metal treated was found to be improved also by the presence of a mild inhibitor such as a lower molecular weight polyhydric aliphatic alcohol, or the corresponding monoethers, although addition of the same is not deemed necessary. Such alcohols as ethylene glycol, glycerol, diethylene glycol, sorbitol and mannitol may be mentioned, as suitable, although the term lower molecular weight is intended to include any polyhydric aliphatic alcohol having from two to about six carbon atoms. Preferably the alcohol is present in amount from about .05 to 1.5% by weight of solution.

The proper hydrogen ion concentration for the acid solution is preferably maintained by the'use HNO:;, this amount corresponds to a volume concentration of from about 5 to 50 cc. per liter.

The fluoride ion is preferably introduced in the form of hydrofluoric acid, although an equivalent amount of any soluble salt of the acid which forms hydrofluoric acid in situ in the solution in the presence of nitric acid may be employed. Ammonium fluoride or acid fluoride may advantageously be used, in which case a proportionately smaller amount of the compound supplying the ammonium ion is employed in order to maintain the NH4+ concentration within the prescribed limits.

It has been found that the fluoride ion concentration should be maintained at a value corresponding to that produced by hydrofluoric acid within the range of from about .01 to about 0.5% by weight of solution. When using 48% HF, this corresponds to a volume concentration of from about .2 to 9 cc. per liter.

Phosphate ion as a critical constituent of the solution advantageously eliminates pitting, prevents smut due to impurities, and increases the brightening effect. The phosphate is preferably introduced as phosphoric acid, that is, orthophosphoric acid. However,. the phosphate ion may be introduced as a soluble phosphate salt, for example, the various ammonium phosphates. Where ammonium phosphate is used it may supply all of the required NH4+ ion or additional ammonium ion may be added in the form of another ammonium compound depending upon the relative concentrations of P04 and NH4+ desired. The amount of phosphate preferred corresponds to that produced by from about .05 to 1% orthophosphoric acid.

A part or all of the ammonium ion content of the solution, as above indicated, may be supplied in conjunction with the phosphate ion or the fluoride ion. However, where these ions are present in the lower range of the recommended concentrations, it may be advisable to add additional ammonium ion. Thus, all of the NH4+ ion or the balance required may be introduced in the form of another ammonium compound, such as the hydroxide or nitrate. Ammonium hydroxide is preferably employed. The preferred amount of the ammonium constituent or of the solution is equivalent to about .01 to about 0.5% ammonia by weight of the solution.

Thus, the bright dip composition of the present invention comprises an aqueous acid solution containing from about .5 to 5% nitric acid by weight, fluoride, phosphate and ammonium ions in amount corresponding to that produced by a solution containing from about .01 to .5% hydrofluoric acid, about .05 to 1% orthophosphoric acid, and from about .01 to 0.5% ammonia with or without about .05 to 1.5% of a lower molecular weight polyhydric aliphatic alcohol by Weight of solution, and with or without cations of a metal electropositive to aluminum in amount corresponding to not less than about .001 by weight of a salt of the selected metal.

Excellent brightening results are obtained when the hydrogen, fluoride, phosphate and ammonium ion concentrations are maintained at any combination of values corresponding to that produced in a solution prepared from nitric acid, hydrofluoric acid, orthophosphoric acid, and ammonia in any combination of amounts within the ranges above stated or any combination of equivalent substances which will produce a solution of substantially the same chemical composition.

'It is'to be'noted that'a'satisfactory degree of brightening. is obtained when using any of the constituents in smaller or larger amounts than indicated above. However, the desired effect is more readily obtained by operating within the stated ranges.

The chemical bath may be operated at temperatures of from about 100 F. to the boiling point. The desired brightening is accomplished with a shorter immersion time at the higher temperatures of from about 190 F. to boiling, and accordingly, this range is preferred. The time of immersion of the articles being treated in the bath should be sufficient to produce the desired brightening, yet insuflicient to cause any undue etching of the metal. It is, of course, dependent primarily on solution temperatures and concentrations and may be widely varied. An immersion time of from about one-half to about ten minutes is recommended.

As a specific example, not intended to limit the invention, maximum brightness was obtained with aluminum and aluminum alloys treated by a five minute immersion in a solution (at a temperature of 190 F. to boiling) prepared by addition of the indicated compounds.

Example I Per Cent Weight of Solution Weight Concentration, g./liter Volume Concen- Oonsmuent tration, cc./liter 30 (70% HNOs) 5 (48% HF) I 1 04 Water, Balance.

Emmple II Weight Volume Ooncen- Concentration, cc./liter tration,

g./liter Per Cent Weight of Solution Constituent HNO; 30 (70%11Noi) a s (48% HF) I I4 Water, Balance.

1 Calculated as N H5.

Eazample III In the following bath, copper ions were introduced in the indicated amount and resulted in an increase in specular brightness, the aluminum alloy sample being treated having a copper content below 0.06% by weight.

Weight Concentration,

g./liter Per Cent Weight of Solution Volume Concen- Constituont tretion, cc./litcr 25 (70% HNO3). 5 (48% HF) 13 (28% NH!) 3 (85% H3PO4), 5

Oats-Jew 0 ca b-l rmNpmeHiO. 0. 015- Waten Bnlance.

1 Calculated as NHa.

1y to immerse the article in the heated solution for the required time. The article is thoroughly water rinsed and dried. However, it is advantageous that prior to immersion the metal is cleaned or degreased by the use of a mild, inhibited alkaline cleaner, for example, Kelite Aviation Cleaner (KDL #1). This is preferably accomplished by immersion in a heated solution of the cleaner for a few minutes, for example, two minutes in solution at 180-200 F., followed by a thorough rinsing prior to introduction into the bright dip solution.

Agitation of the bath is not essential, but is recommended since it produces more uniform results and tends to decrease the required immersion time. Mechanical or air agitation may be used, but the former is preferable since it does not cool the solution as does air agitation.

The process may be operated on a continuous or semi-continuous basis, or it may be conducted as a batch process. In case of the former, the components are gradually depleted and calculated additions of the compounds supplying these ions are periodically added to maintain proper concentrations.

' changers, or, if electrical heating is desired Karbate shielded immersion heaters may be employed.

The results obtained with the chemical bright dip of the present invention do not produce a surface smoothing efiect in comparison to mechanical buffing or electropolishing. However, the surface luster or brightness compares very favorably with that produced by electrobrightening, while num or alloy articles with the bright dip is merethe chemical bright dip requires only a short immersion time and no electric current. In short, the improved chemical bath produces the desired result much more economically.

I claim:

1. A composition for the chemical brightening of aluminum and aluminum alloys comprising an acid aqueous solution containing hydrogen, nitrate, fluoride, ammonium, and phosphate ions in concentration corresponding to that produced by a solution containing from about .5 to 5% nitric acid, about .01 to .5% hydrofluoric acid, about .01 to 0.5% ammonia and about .05 to 1% orthophosphoric acid by weight of the solution.

2. A composition according to claim 1 containing also cations of a metal more electropositive than aluminum.

3. A composition for the chemical brightening of aluminum and aluminum alloys comprising an acid aquous solution containing hydrogen, nitrate, fluoride, ammonium, and phosphate ions in concentration corresponding to that produced in a solution containing from about .5 to 5% nitric acid, about .01 to .5% hydrofluoric acid, about .01 to 0.5% ammonia and about .05 to 1% ortho- -phosphoric acid by weight of the solution, and

from about .05 to 1.5% by weight of a compound selected from the group consisting of lower molecular weight polyhydric aliphatic alcohols and ethers thereof.

4. A composition according to claim 3 containaqueous solution containing from about .5 to 5 nitric acid, about .01 to 0.5% hydrofluoric acid, about .01 to 0.5% ammonia, and from about .05 to 1% phosphoric acid by weight of solution.

6. A composition according to claim 5 containing cations of a-metal electropositive to aluminu'm in amount corresponding to that produced by at least 001% of a salt of the metal.

7. A composition for the chemical brightening of aluminum and aluminum alloys comprising an aqueous solution containing by weight from about .5 to 5% nitric acid, about .01 to 0.5% hydrofluoric acid, about .01 to 0.5% ammonia, and from about .05 to 1% phosphoric acid, and from about .05 to 1.5% by weight of a lower molecular weight polyhydric aliphatic alcohol. I I

" 8. A composition-according to claim 1 in which thesaid phosphate is orthophosphoric acid.

9. An aqueous solution for chemically brightening aluminum and aluminum alloys consisting essentially of about .5 to 5% nitric acid, about .01 to 0.5% hydrofluoric acid, about .01 to 0.5% ammonia, about .05 to 1% orthophosphoric acid by weight.

10. A composition according to claim 9 containing at least .001 by weight of copper nitrate 3o calculated as omNom- 3H2O.

11. A process for the chemical brightening of aluminum and aluminum alloys which comprises treating the metal in an aqueous acid bath containing hydrogen, nitrate, fluoride, ammonium and phosphate ions in concentration corresponding to that produced by a solution containing from about .5 to 5% nitric acid, about .01 to 0.5% hydrofluoric acid, about .01 to 0.5% ammonia, and

about .05 to 1% phosphoric acid by weight, and maintaining the solution at a temperature of about F. to the boiling point.

12. A process according to claim 11 in which the bath also contains from about .05 to 1.5% by weight of a lower molecular Weight polyhydric alcohol.

13. A process according to claim 11 in which the bath also contains cations of a metal electropositive to aluminum.

14. A process for the chemical brightening of aluminum and aluminum alloys which comprises immersing the metal in a heated aqueous bath containing about .5 to 5% nitric acid, about .01 to 0.5% hydrofluoric acid, about .01 to 0.5% ammonia, from about .05 to 1 orthophosphoric acid and about .05 to 1.5% glycerol by weight for a time sufficient to brighten the metals but insufiicient to cause overetching thereof, maintaining the bath at a temperature of from about F. to the boiling point, Withdrawing the treated metal and removing adhering solution.

15. A process according to claim 14 in which the bath also contains not less than about .001% by weight of copper nitrate calculated as Cu(NO3) 2- 3H2O FREDERICK HAROLD HESCH.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,994,500 Boller Mar. 19, 1935 2,365,153 Stevens Dec. 19, 1944 2,393,875 Van Dusen Jan. 29, 1946 2,446,060 Pray July 27, 1948 

11. A PROCESS FOR THE CHEMICAL BRIGHTENING OF ALUMINUM AND ALUMINUM ALLOYS WHICH COMPRISES TREATING THE METAL IN AN AQUEOUS ACID BATH CONTAINING HYDROGEN, NITRATE, FLUORIDE, AMMONIUM AND PHOSPHATE IONS IN CONCENTRATION CORRESPONDING TO THAT PRODUCED BY A SOLUTION CONTAINING FROM ABOUT .5 TO 5% NITRIC ACID, ABOUT .01 TO 0.5% HYDROFLUORIC ACID, ABOUT .01 TO 0.5% AMMONIA AND ABOUT .05 TO 1% PHOSPHORIC ACID BY WEIGHT, AND MAINTAINING THE SOLUTION AT A TEMPERATURE OF ABOUT 100*F. TO THE BOILING POINT. 